docs: def get_variable_for_feature(self, feature_key, variable_key):
      """ Get the variable with the given variable key for the given feature.
  
      Args:
        feature_key: The key of the feature for which we are getting the variable.
        variable_key: The key of the variable we are getting.
  
      Returns:
        Variable with the given key in the given variation.
      """
      feature = self.feature_key_map.get(feature_key)
      if not feature:
        self.logger.error('Feature with key "%s" not found in the datafile.' % feature_key)
        return None
  
      if variable_key not in feature.variables:
        self.logger.error('Variable with key "%s" not found in the datafile.' % variable_key)
        return None
  
      return feature.variables.get(variable_key)